The present invention relates to adhesive tapes and to methods of making and using the same. More particularly, the present invention contemplates adhesive tapes useful to detachably join elements together, especially at least one flexible element. The tapes are especially useful in printing applications for detachably mounting printing plates in printing presses during printing operations.
In flexographic printing, flexible printing plates carrying the image to be printed are mounted to printing press cylinders for resilient printing contact with substrates to be printed. The printing plates are formed of polymeric materials such as rubber or photopolymer, and they are removably mounted to the steel print cylinders by double sided tape sometimes referred to as "stickyback." In printing applications for both labelling and packaging, the printing plates are reused and several different plates may be used in daily production runs of different products.
The resilient contact between the printing plate and substrate to be printed is provided at least in part and/or significantly affected by the compressibility or resiliency of the tape. It is known in the art to use a double sided tape having a carrier containing a flexible foam layer and opposed layers of pressure sensitive-adhesive. Such double coated foam carrier tapes generally provide sufficient compressibility to assure printing of relatively fine or small indicia. However, the foam carrier tapes tend to be too compressible for printing of relatively large indicia (e.g. indicia having border dimensions of one-eighth inch or more) and cause excessive ink flow on the substrate. For printing such large indicia, the prior art teaches the use of a less compressible tape having a carrier containing a thermoplastic resin layer such as a polyvinylchloride film layer. Such vinyl carrier tapes tend to provide satisfactory printing of large indicia, but they do not provide the sharpness and fine detail needed in relatively small indicia. Consequently, printers have tended to inventory and use both foam and vinyl carrier tapes in order to attain satisfactory print quality for the full size range of print indicia.
In order to obtain uniform print quality, it is necessary that the tape be of uniform caliper or thickness. Variations in tape thickness tend to cause print color or intensity differences. Foam carrier tapes have tended to have unacceptable thickness variations, for example, a 20 mil thick tape may range from 16 to 24 mils in thickness. Such thickness variations yield undesirable print color or intensity differences as noted above. In extreme cases, thickness variations may prevent proper end-to-end abutting alignment of the printing plate around the print cylinder. For example, the two abutting tape edges may be at sufficiently different heights to cause a detectable printing difference across the butt joint.
The ease of tape removability from the print cylinder is important to efficient use of the tape and to maintaining good printing production rates. During installation of the printing plate, it may be necessary to remove and reposition the tape relative to the print cylinder. If the tape tears during its removal for repositioning, the tape is wasted and discarded without printing use. Following the printing process, the tape is removed from the print cylinder in preparation for the mounting of a different printing plate. In both cases, the tape should strip cleanly and separate from the print cylinder by hand manipulation without tearing or rupturing internal layers so as to leave behind irregular pieces of tape residue such as adhesive and/or tape fragments. Preferably, the tape pieces should strip from the cylinder as an integral piece without leaving behind tape residue and thereby provide "one piece removability". Prior foam carrier tape constructions tended to shear or tear in the foam layer during removal from the cylinder and required additional time for tape removal and clean-up processing.
G.B. 1,533,431 discloses a laminated double sided tape for mounting flexographic letterpress printing plates including an elastomeric layer having rigid-walled frangible bubbles, a dimensionally stable reinforcing layer and opposed outer adhesive layers. The frangible bubbles are broken with pressure and the wall fragments are retained in the resulting voids.
A double sided tape for mounting a flexible printing plate to a drum is disclosed in U.S. Pat. No. 4,574,697. The tape includes a polyurethane flexible foam applied to a polyethylene terephthalate base film and outer adhesive layers. The tape is indicated to be removable from the printing plate and drum as an integral sheet, and to be an improvement over vinyl carrier tapes that lack memory and do not retain gauge during long printing runs. A similarly constructed tape having a polyethylene foam layer selected to reduce vibrations is disclosed in European Patent Application 0 206 760.
U.S. Pat. No. 3,983,287 discloses a printing blanket comprising a laminate including a compressible interior support layer of incompressible elastomer having dispersed and bonded therein frangible rigid-walled bubbles that have been broken to impart compressibility.